Conventionally, there have been proposed various rechargeable secondary batteries that are mainly used as a power source for a portable device. In addition, in recent years, from the standpoint of environmental considerations, development has been conducted on high-capacity secondary batteries to be used for load leveling in natural energy power generation such as wind power generation or solar power generation, or to be used as a power source for a vehicle such as an automobile or electric train. In the field of natural energy, secondary batteries are used mainly for equalizing energy generation with energy supply. In a case where a secondary battery is installed in a vehicle, regenerative electric power that is generated when the brake is applied is stored in the in-vehicle secondary battery, and the stored electric power can be used as a source of power for the vehicle. Accordingly, the operating energy efficiency of the vehicle can be improved. For example, a nickel-metal hydride secondary battery, which is one of the types of alkaline storage batteries, is considered to be suitable as such an in-vehicle secondary battery when various conditions such as energy density, load following capability, durability, and manufacturing cost are taken into account (see Patent Literature 1, for example).
Secondary batteries for industrial use such as in-vehicle secondary batteries are required to have very high durability. Nickel-metal hydride secondary batteries are commonly used as industrial use secondary batteries. One of the factors that cause degradation in the charge/discharge performance of a nickel-metal hydride secondary battery is an increase in the internal resistance of the battery. Nickel hydroxide (Ni(OH)2), which is used as a main active material of the positive electrode of a nickel-metal hydride secondary battery, is oxidized at the time of charging and thereby transforms into nickel oxyhydroxide (NiOOH). Nickel hydroxide is an insulating material whereas nickel oxyhydroxide is a semiconducting material. Therefore, the higher the state of charge (SOC) of a secondary battery becomes, the lower the internal resistance of the secondary battery becomes.
It is necessary to maintain electrical conductivity within the electrode (positive electrode) even when the SOC is low. For this reason, nickel hydroxide coated with cobalt hydroxide (Co(OH)2) serving as a conductive agent has been put in practical use as a positive electrode active material (see Patent Literature 2, for example). As shown in FIG. 9, Patent Literature 2 discloses that cobalt hydroxide particles are added in such a manner that the surface of aggregates (secondary particles) of nickel hydroxide particles (primary particles) is coated with the cobalt hydroxide particles. When oxidized at the time of charging of the secondary battery, the cobalt hydroxide transforms into electrically conductive cobalt oxyhydroxide (CoOOH). In the lower right image in FIG. 9, a white portion seen on the surface of a black aggregate (secondary particle) of nickel hydroxide particles is a cobalt compound. In FIG. 10, a substantially spherical large-diameter particle is an aggregate (secondary particle) of nickel hydroxide particles, and particles in a crushed state that surround the aggregate are nickel hydroxide particles (primary particles). Reduction of cobalt oxyhydroxide does not occur in a normal working voltage range of a secondary battery. Therefore, a certain level of electrical conductivity can be maintained within the electrode (positive electrode) even when the SOC is low.